1. Field of the Invention
The invention relates to an X-ray imaging device and particularly relates to the backup time of an X-ray imaging device having an automatic exposure control (phototimer) function.
2. Description of Related Art
An X-ray imaging device, for example, is formed by an X-ray tube that irradiates a subject by X ray; an X-ray detector including a flat panel detector (FPD) for imaging the X ray that passes through the subject; a phototimer that automatically controls exposure time of the X ray; a measurement unit for measuring the body thickness of the subject; and a control console having an operating panel for operating the foregoing components.
The X ray that penetrates the subject is converted into an electrical signal by a phototimer detector. The phototimer includes an integrator for integrating the electrical signal; and a comparator for comparing an output signal of the integrator that is in proportion to concentration with a concentration reference signal. In phototimer imaging, an X-ray shutoff signal is outputted from the comparator to stop X-ray exposure when the output signal of the integrator is greater than the concentration reference signal, so as to complete the X-ray imaging with proper exposure. Even though the imaging time can be automatically determined to achieve proper exposure, if the set backup time is shorter than the imaging time, the X-ray exposure is stopped due to the backup time.
During phototimer imaging, for instance, the phototimer detector may detect X ray that mainly passes through the bones of the subject. As a consequence, the electrical signals inputted into the integrator become fewer, and the time for the output signal of the integrator to exceed the concentration reference signal becomes longer than the backup time. In that case, the X-ray exposure is stopped due to the backup time to prevent overexposure.
Furthermore, an X-ray imaging device has been provided (see Reference 1, for example), which automatically sets the tube voltage, the tube current, and the imaging time based on the measured body thickness of the subject and does not require the operator to input imaging parameters according to different subjects. With the help of the X-ray imaging device, inexperienced users can also operate the device to obtain acceptable imaging results. In addition, mistakes in inputting imaging parameters and unnecessary X-ray exposure can be prevented, and an image that correctly matches the imaging parameters can be obtained.
Reference 1: Japanese Patent Application Laid-open No. H06-277204